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Abstract

Cell assemblies are considered to be physiological as well as functional units in
the brain. A repetitive and stereotypical sequential activation of many neurons was
observed, but the mechanisms underlying it are not well understood. Feedforward networks,
such as synfire chains, with the pools of excitatory neurons unidirectionally
connected and facilitating signal transmission in a cascade-like fashion were proposed
to model such sequential activity. When embedded in a recurrent network, these were
shown to destabilise the whole network’s activity, challenging the suitability of the
model. Here, we investigate a feedforward chain of excitatory pools enriched by inhibitory
pools that provide disynaptic feedforward inhibition. We show that when
embedded in a recurrent network of spiking neurons, such an augmented chain is capable
of robust signal propagation. We then investigate the influence of overlapping
two chains on the signal transmission as well as the stability of the host network. While
shared excitatory pools turn out to be detrimental to global stability, inhibitory overlap
implicitly realises the motif of lateral inhibition, which, if moderate, maintains
the stability but if substantial, it silences the whole network activity including the signal.
Addition of a disinhibitory pathway along the chain proves to rescue the signal
transmission by transforming a strong inhibitory wave into a disinhibitory one, which
specifically guards the excitatory pools from receiving excessive inhibition and thereby
allowing them to remain responsive to the forthcoming activation. Disinhibitory circuits
not only improve the signal transmission, but can also control it via a gating mechanism.
We demonstrate that by manipulating a firing threshold of the disinhibitory neurons,
the signal transmission can be enabled or completely blocked. This mechanism
corresponds to cholinergic modulation, which was shown to be signalled by volume
as well as phasic transmission and variably target classes of neurons. Furthermore,
we show that modulation of the feedforward inhibition circuit can promote generating
spontaneous replay at the absence of external inputs. This mechanism, however, tends
to also cause global instabilities.
Overall, these results underscore the importance of inhibitory neuron populations
in controlling signal propagation in cell assemblies as well as global stability. Specific
inhibitory circuits, when controlled by neuromodulatory systems, can robustly guide or
block the signals and invoke replay. This mounts to evidence that the population of interneurons
is diverse and can be best categorised by neurons’ specific circuit functions
as well as their responsiveness to neuromodulators.